I use either my index finger (ie. the 2nd finger) or my thumb.. I find I use my thumb the most but I have no trouble doing a PH with my index finger if the pick is 0.50mm or more..

Basically it's the exact same method as the thumb except in reverse. The picking direction is upward and you angle it towards the left as you pick upwards so that the fleshy part of the index finger catches the string on the way through.. just like the way the thumb method is a downward stroke with a slight angle to the right, so that the fleshy part of the thumb catches the string on the way through.. I think the thumb PH is much more reliable.. but either way works..